使 for 循环只运行一次的花括号？

Question

Let's say I have a pseudocode and sum = 3

for i = (1 to 3)
    sum = sum + i
    return sum

This returns 9 because 3+1=4, 4+2=6, 6+3=9

---

But when doing this

for i = (1 to 3) {
    sum = sum + i
    return sum
}

Sum returned is 4? Why does the curly braces somehow make the for loop run only once?

Answer 1

If you have the following C code:

for (int i=1; i<=3; ++i)
   sum = sum + i;
   return sum;

Then you have a misleading way of writing

for (int i=1; i<=3; ++i)
   sum = sum + i;

return sum;

That's because the above snippets are both equivalent to the following:

for (int i=1; i<=3; ++i) {
   sum = sum + i;
}

return sum;

The body of a for loop is either a single statement or a block. Most languages don't consider indentation level to be significant. (Python is a notable exception.)

In contrast, the following executes the return each pass of the loop instead of after the loop is complete:

for (int i=1; i<=3; ++i) {
   sum = sum + i;
   return sum;
}

Well, it'll never reach the second pass of the loop, of course.

Answer 2

Your indentation of the first loop is misleading. Unroll both and it will be apparent what's going on:

sum = sum + 1
sum = sum + 2
sum = sum + 3
return sum

This one adds 6 to sum and then returns it.

sum = sum + 1
return sum
sum = sum + 2
return sum
sum = sum + 3
return sum

This one adds 1 to sum and then returns it. A return statement ends execution of the function (even if there's more statements after it, or it's inside a loop).

Answer 3

    sum = sum + i
    return sum

run sum = sum + i 3 times. Then execute return sum , cause if loop construction has no braces it will run only one next operation.

Here

for i = (1 to 3) {
    sum = sum + i
    return sum
}

every loop iteration will try to execute return sum and only first iteration will really execute this. Next iteraions will not be executed cause return will break the loop.

Answer 4

In the second example, you are including a return statement into the body of the for loop, and thus returning on the first iteration. In the first example, only the addition/assignment statement is executed in your loop and then the result returned.